Data reduction techniques usually adopt hierarchical classifications to reduce numerous data. In the database disclosed in Non-Patent Document 1, for example, hierarchical classifications referred to as MeSH terms are assigned to documents. In the patent database operated by the Patent Office, for example, a plurality of hierarchical classifications such as F terms is assigned to patent documents. Hereinafter, hierarchical classifications assigned to data will be referred to as classification hierarchy.
Non-Patent Document 2 discloses a relevant art that allows users to read documents assigned with the aforementioned hierarchical classifications. Non-Patent Document 2 discloses a system, referred to as OLAP, which extends multifaceted data display methods so as to achieve high-speed processing with respect to a very large hierarchy. In the relevant art disclosed in Non-Patent Document 2, users are each allowed to select a classification so as to display data quantity included in the classification at a high speed. In the relevant art disclosed in Non-Patent Document 2, users are each allowed to select a classification on a vertical axis and a classification on a horizontal axis so as to display the result by way of a table form.
Owing to the aforementioned operation, the relevant art disclosed in Non-Patent Document 2 is able to display a list of documents assigned with a plurality of classifications. Hereinafter, a group of classifications used for displaying data will be referred to as a classification axis.
In the case of a very large classification hierarchy, however, it is difficult for users to determine which classification needs to be selected. As documents used in the system disclosed in Non-Patent Document 2, for example, there are about five-hundred thousand documents and about thirty-four hundred thousand classifications. Therefore, it is very difficult for users to select display-wished classifications within the classification hierarchy.
Non-Patent Document 3 discloses a relevant classification selecting method. Non-Patent Document 3 discloses retrieval-resultant document lists via document retrieval along with a method of displaying classification axes related to retrieval-resultant document lists. According to the method disclosed in Non-Patent Document 3, keywords are input to retrieve documents so as to display retrieval-resultant documents while a plurality of classifications pre-assigned to retrieval-resultant documents is displayed as a classification axis. When displaying the classification axis, each classification needs to be selected because the display area is limited.
In addition, Non-Patent Document 3 discloses a method of selecting an upper-limit place among classifications in an order counting from a classification assigned to a larger number of retrieval-resultant documents and a method of selecting a combination of classifications which is able to display the largest number of retrieval-resultant documents. Furthermore, Non-Patent Document 3 discloses a method of minimizing cost such as the number of times a mouse needs to be clicked to display all the content.